Children of recently immigrated Latinos are at increased risk for exposure to multiple social and environmental factors that also contribute to inadequate nutrient intake, disadvantaging their development. Folate, a key methyl donor critical for DNA synthesis, methylation, and neurodevelopment, can be ingested as naturally occurring food folate or as synthetic folic acid (FA), present in fortified grains and vitamin supplements. Cost and convenience lead to a predominance of fortified foods in accessible staples for poor Latino immigrants. In rodent pups, a post-natal diet of insufficient folate impairs normal neurodevelopment, while diets with excess FA impair short term memory and behavior. In adult humans, excess FA intake exacerbates memory loss and detrimental cognitive effects of vitamin B12 deficiency. Data examining toddler folate intake and cognition are limited. A 2015 National Toxicology Program (NTP) expert panel identified study of cognitive impact of high FA intake during toddlerhood as a priority. The prevalence of neurocognitive and behavior problems in low-income toddlers paired with concerns from rodent diet data suggest the need to better account for dietary intake in poor toddlers, including examining high FA intake from fortified foods. Data on diet and folate biomarkers in US Latino toddlers are sparse. Our analysis of NHANES data suggests risk for consuming insufficient folate (below the Estimated Average Requirement) and also risk for ingesting excess FA (above the tolerable Upper Limit for age) are both significantly increased in toddlers of Spanish preference Latinos, though few take supplements. Latino toddlers may be susceptible to low folate or high FA intake because of highly prevalent polymorphisms that exacerbate the impact of low folate (MTHFR; rs1801133) or high FA (DHFR; rs70991108) intake. In our ongoing pilot of 2 year olds (N=55) documenting diet and neurodevelopment in Columbia?s Early Head Start, most mothers are immigrants from central Mexico who maintain pre-migration dietary habits when feeding their US born toddlers, while including foods provided by government assistance. We have collected toddler blood samples, data on food insecurity, migration, acculturation, home cognitive stimulation, repeated 24 hour diet recalls and neurodevelopmental assessments. We hypothesize toddlers may over-consume fortified grains such as boxed cereal, bread, and traditional foods made with fortified corn flour. At 24 months, 25% of toddlers have high plasma folate, while 24 month diet intake of synthetic FA increases with crowded living conditions and correlates inversely with executive function and behavior assessed at ages 30 and 36 months suggesting a potential negative association with FA consistent with rodent data. We propose to expand on our innovative pilot, addressing an NTP identified priority, examining diet and neurodevelopment in 145 toddlers (adding 90 new recruits to the existing 55), followed longitudinally from 24 to 36 months measuring cognition, behavior, diet intake of folate, FA, and B12, serum and red cell folate and B12, and risk modifying polymorphisms. Our comprehensive study in Latino migrants may inform policy to improve at-risk toddlers? developmental outcomes